The present invention relates to a pulverized coal burner device capable of ensuring stable combustion even when mill load is low.
Coal pulverized by a mill is entrained in a stream of primary air so as to be burned in a pulverized coal burner device. The primary air carrying the pulverized coal must flow at a flow rate beyond some lower limit in order that the pulverized coal can be entrained in the primary air even when the mill load is lowered and supply of the pulverized coal from the mill is decreased. Therefore, in response to reduction of the mill load, a ratio A/C between the quantity (A) of the primary air and the quantity (C) of the pulverized coal becomes high. Meanwhile, it is preferable that the ratio A/C is less than 2.5 or 3.0 or so in order to ensure stabilized combustion in the pulverized coal burner. Turndown (limit of combustion with a lower load) of the pulverized coal burner is on the order of 40% of the mill load.
Therefore, for low mill load, there has been devised and demonstrated a device as shown in FIG. 1 which utilizes a system as used in coal-burning boilers. More specifically, pulverized coal entrained in a stream of primary air flowing from a mill a is separated from the primary air in a cyclone b so that the ratio A/C becomes a predetermined value; the separated pulverized coal is injected through a burner c into a furnace d and is burned while the primary air substantailly free from the pulverized coal is forced into the furnace d through a port e opened on the upper portion of the furnace d, whereby a slight quantity of pulverized coal in the primary air is burned.
With the above-described device and in the case of low mill load, the upper portion of the furnace d is so low in temperature that the pulverized coal injected through the port e into the furnace d will not burn and consequently combustion efficiency is decreased. Furthermore, there is a serious problem that the unburned pulverized coal may trigger an explosion of coal dusts downstream of the furnace. Moreover, temperature of the primary air carrying the pulverized coal is too low to be used as secondary air.
In view of the above, a primary object of the present invention is to provide a pulverized coal burner device capable of ensuring stabilized combustion even in the case of low mill load.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.